As Diamond sang the lyrics "good times never seemed so good," Clooney jumped in with the "so good, so good" response that fans have added over the years. The actor appeared to be having a good time while giving a mock-serious delivery of the lyrics.

"When he was growing up, he thought he'd be a singer because his aunt (Rosemary Clooney) was such a big singer," she told The Hollywood Reporter. "But when he went into the studio, people just stared at him. … It's clear he's a better actor and director than a singer. And on this night he was also a good sport."

Many in attendance cheered on Clooney and recorded the once-in-a-lifetime event on their cameras and cellphones, posting the videos on YouTube. (Watch one below.)

At the event, Clooney was presented with the Brass Ring award for his humanitarian efforts around the world. Presenter Shirley MacLaine called him the “most generous, sweetest, charming human being I know.”